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A Super Nonlinear Electrodeposition-diffusion-controlled Thin Film Selector Xinglong Ji, Li Song, Wei He, Kejie Huang, Zhiyuan Yan, Shuai Zhong, Yishu Zhang, and Rong Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17235 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

A Super Nonlinear Electrodeposition-diffusion-controlled Thin Film Selector Xinglong Ji1, Li Song1, Wei He1, Kejie Huang2, Zhiyuan Yan1, Shuai Zhong1, Yishu Zhang1 and Rong Zhao*1 1

Department of Engineering Product Design, Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore E-mail: [email protected] Prof. K. Huang 2 Institute of Information and Communication Engineering, Zhejiang University, Hangzhou, 310027, China Keywords: thin film selector, Ag-alloyed Ge2Sb2Te5, high nonlinearity, low leakage, electrodeposition-diffusion-controlled dynamics Abstract Selector element with high nonlinearity is an indispensable part in constructing high density, large-scale, 3-D stackable emerging non-volatile memory (NVM) and neuromorphic network. Although significant efforts have been devoted to developing novel thin-film selectors, it remains a great challenge in achieving good switching performance in the selectors to satisfy the stringent electrical criteria of diverse memory elements. In this work, we utilized highdefect-density chalcogenide glass (Ge2Sb2Te5) in conjunction with high mobility Ag element (Ag-GST) to achieve a super nonlinear selective switching. A novel electrodepositiondiffusion dynamic selector based on Ag-GST exhibits superior selecting performance including excellent nonlinearity (< 5 mV/dev), ultra-low leakage (< 10 fA), and bidirectional operation. With solid microstructure evidence and dynamic analyses, we attributed the selective switching to the competition between the electrodeposition and diffusion of Ag atoms in glassy GST matrix under electric field. A switching model is proposed, and the indepth understanding of the selective switching mechanism offers an insight of switching dynamics for electrodeposition-diffusion-controlled thin-film selector. This work opens a new direction of selector design by combining high mobility elements and high-defect-density 1

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chalcogenide glasses, which can be extended to other materials with similar properties. 1. Introduction With excellent scalability and 3-D stackable feature, cross-point architecture is commonly adopted for various emerging non-volatile memory (NVM) and neuromorphic network implementation.1–4 However, sneak path problem remains as the main obstacle for this architecture towards large-scale and high density applications.5,6 A highly nonlinear selector device is required at each cross-point, which can deliver the right voltage and current to the memory (or memristor) cells only when the voltage applied exceeds a certain threshold value. Besides exhibiting good selective performance, the selector should also match the corresponding memory elements’ characteristics, especially the strict voltage and ON/OFF ratio matching demands.7 Such stringent requirements place a significant challenge to develop a selector that can be applied to diverse NVM arrays.8,9 Experiencing the drawbacks on poor scalability, complicated epitaxy process, the well-developed Si-based selectors can hardly be further promoted for high density, 3-D stackable cross-point chips.10-14 This has motivated active researches on thin-film selector that can be scaled down to smaller size and fabricated at back-end-of-line (BEOL). In recent years, various selector devices with different mechanisms have been proposed, including field assisted superlinear threshold (FAST)15, Ovonic

threshold

switch

(OTS)16,

Mixed-ionic-electronic-conduction

(MIEC)17,

electrochemical metallization (EM), etc. Among these candidates, EM based selector, which is based on metal cation transport and redox reactions in metal-insulator-metal (MIM) structure, is one of the outstanding representatives and has shown attractive switching performance.18–21 However several critical properties, such as selectivity, nonlinearity and leakage current, are still on the weak side comparing with Si-based selectors, which are insufficient for practical applications. New strategy and an in-depth understanding of the 2


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selective switching mechanisms to guide the design of EM based selector are highly needed for ultra-large-scale memory array implementation. In this paper, we designed and implemented an electrodeposition-diffusion dynamic controlling selector using high ion-mobility element Ag and high vacancy glassy chalcogenide material Ge2Sb2Te5 (GST). Here, we intentionally heavily doped Ag into GST to promote the Ag dynamics leading to a highly nonlinear selective switching. The Ag-GST selector demonstrates excellent selecting performance of ultra-low leakage current (< 10 fA), high nonlinearity (< 5 mV/dec), large hysteresis window, bidirectional operation and large ON/OFF ratio (> 109), showing great potential as an alternative to Si-based selective devices. We further performed microstructure characterization and dynamic analyses of the Ag migration process in glassy GST under different electric fields to provide direct evidences and in-depth understanding of selective switching dynamics. The findings reveal that the selective switching is associated to the competition between the electrodeposition and diffusion of Ag atoms in glassy GST matrix under electric field. A switching model based on migration and redistribution of Ag is proposed. This work provides a guideline for selector design by utilizing the high mobility elements and high-defect-density chalcogenide glass, paving the way for developing high density cross-point arrays.

2. Experimental Section Vertical Device Fabrication: A vertical device structure was adopted for the selector device, as shown in Figure 1. Firstly, 60 nm TiW film was deposited on the Si/SiO2 substrate by direct current magnetron sputtering, serving as bottom electrode. Secondly, 60 nm SiO2 film was deposited using radio frequency sputtering method, which isolates the bottom electrode from the top electrode. Vias with 4 × 4 µm2 were opened by lift-off method on the isolation layer. The switching layer contacted with the bottom electrode only in the via region. The 3



depth of the via was defined by the thickness of the isolation layer. Then 100 nm Ag-alloyed GST functional layer was co-sputtered over the SiO2 isolation layer using Ag and GST targets. Finally, top electrode (60 nm Pt) was deposited on the switching layer. All the processes were performed at room temperature.

Lateral Device Fabrication: To investigate the diffusive dynamics of Ag ions under electric field, we also fabricated lateral devices. 100 nm Ag-doped GST film was deposited on the Si/SiO2 substrate by co-sputtering method at room temperature. Then e-beam lithography method was employed to form lateral electrode patterns with different gaps. 60 nm Ag film was deposited on the pattern, serving as planar electrodes. The distance between the lateral electrodes varies from 500 nm to 10 µm. Making a trade-off between switching performance and real-time observation convenience, we chose the 6 µm gap to present.

Electrical Measurement: All the electrical characteristics were measured by a Keithley 4200SCS semiconductor characterization system with high-resolution source and measurement units. The various pulse conditions utilized in the testing are also generated by Keithley 4200.

Microstructure Characterization: The cross-section of vertical device was prepared by a dual beam focused ion beam system. To prevent the ion beam distort the physical situation, we used small ion beam current (28 pA) for sample thinning and cleaning. The thickness of the sample was 70 nm. The microstructure characterization is performed by a Tecnai F20 Transmission Electron Microscope (TEM) system at an accelerating voltage of 200 kV. Both bright field images and high angle annular dark field (HAADF) images are acquired for the interested regions. Electron dispersive x-ray spectroscopy (EDX) mapping module was 4


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employed for Ag distribution characterization. The surface topography and chemical composition of conductive filament in planar device were examined by a field-emission scanning electron microscopic equipped with energy dispersive spectroscopy (FESEM-EDS).

3. Result and Discussion A vertical MIM structure was adopted to demonstrate the basic function of the proposed selector, in which Ag-alloyed Ge2Sb2Te5 (Ag-GST) material serving as a switching layer is sandwiched between two inert electrodes (Pt and TiW), as shown in Figure 1(a)–(b). The Pt/Ag-GST/TiW cells were in a high resistance state initially, with resistances varying from 104 to 109 Ω (see Figure S1(a), Supporting Information) of different Ag concentrations (10%, 20% and 30%). Figure 1(c) shows the typical dual-sweep current-voltage (I-V) curves of the Pt/Ag(10%)-GST/TiW selector. During each sweeping, the current maintains constantly low at low voltage region. When the voltage exceeds the threshold voltage (Vth), the current abruptly jumps to a high value, which is clamped by compliance current (Icc). A turn-on slope of < 5 mV/dec is achieved for bipolar sweeping. Gradually reducing the applied voltage, the current drops sharply at a low holding voltage (Vhold) and the selector returns to OFF-state. The difference between Vth and Vhold (above ~0.8 V) forms a large hysteresis window, which allows to avoid the partial reset and enable novel operation scheme without stringent matching requirement.22 It is worth noting that the measurable OFF current in Figure 1(c) is limited by the test precision. A higher precision testing configuration is set up to perform the low electric field characterization. We observe that the leakage currents near Vth/2 are still below the detection limit (< 10 fA) as shown in Figure 1(d), which is the lowest reported so far. With such low leakage currents, the sneak path problem can be handled well. I-V characterization for other Ag concentrations (20%, 30%) was also investigated based on the 5



same device structure. The Vth and OFF state current are found to be highly related to the silver concentration (see Figure S1(b), Supporting Information). When the Ag concentration increases to 20% and 30%, both Vth and OFF state leakage current degenerate, which may relate to the lower initial resistances. As 10% Ag concentration gives the lowest leakage current, the following experiments are based on this configuration. We also performed the I-V sweeping characteristics with different Icc, as shown in Figure S2(a). An ON current up to 10 µA has been demonstrated with no obvious impact on the Vth and Vhold. The greater than 105

A ON current and smaller than 10-14 A leakage current indicate that a selectivity higher than

109 was achieved. We vertically integrated a resistive memory (TiO2) on top of the proposed selector to imitate the 1 selector 1 resistive NVM (1S1R) structure (see Figure S2(b)). The selector maintains a high ON/OFF ratio, though the ON current slightly decreases for both bias directions when the resistive memory is in high resistance state. This high selectivity could enable the construction of large-scale cross-point array.8 According to Vth/2 schema, memory array in the scale of TB is achievable.8 To further demonstrate the practicability of proposed selector, we fabricated a 6 × 6 pure selector cross-point array (shown in Figure S3, supporting information). The narrow threshold voltage distributions indicate good device to device variability. We further investigated the selector at pulse operation mode. Figure 2(a)-(f) illustrate the selective switching characteristics at different pulse width (100 µs, 10 µs, and 1 µs) and pulse amplitude (from 2.0 V to 3.2 V) for both bias directions, respectively. The results indicate that the selector can realize reversible selective switching during pulse operation. As shown in Figure 2, with pulse widths of 100 µs, 10 µs, and 1 µs, the selector was turned on at 2.1 V, 2.2 V, 2.4 V for positive operation and -2.2 V, -2.4 V, -2.7 V for negative operation, respectively. This implies that faster turn on speed can be achieved at slightly higher pulse amplitude. The 6


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ON voltages of negative direction are slightly higher than those of positive direction, consisting with the observation in DC sweeping. This may be caused by the asymmetric work functions between the top electrode (TE) and bottom electrode (BE), which, however, will not affect the function verification.23 Reversible switching without degradation up to 105 cycles for positive direction and 3×104 cycles for negative direction are demonstrated, respectively, as shown in Figure 3(a). The pulses we applied for endurance characterization are 10 µs, 2.5 V for positive direction and 10 µs, 2.6 V for negative direction, respectively. It should be noted that the OFF current cannot be read on-the-fly during pulse mode due to the limit of parameter analyzer while the readings of ON-state current and ON Vth are accurate. Thus, DC sweeping after each batch of endurance test was performed to amend the OFF-state current. The I-V sampling during cycle-to-cycle test is shown in Figure 3(b). The repeatable switching behavior and the nearly constant Vth during 104 cycling indicate that the proposed selector device is robust and reliable for practical application. The ultra-low leakage and large ON/OFF ratios of both directions also maintain well during the endurance testing. The excessive cycling induced failure exhibits as a shorting failure. After breakdown, the ON current increased more than one order of magnitude and the device stayed at a low resistance state. However, surprisingly, the selection characteristic of device could be fully recovered by a thermal annealing at 80oC for 12 hours (see Figure S4, supporting information). This indicates that the breakdown may associate to the formation of metallic conductive filament between the TE and BE. The thermal treatment can accelerate the diffusion of metallic filament, resulting in the recovery of the selective switching behavior. The mechanism of selective switching is the subject of great current interest. Ag-GST with light doping concentration (< 5%) has been well studied for NVM applications, in which the phase transition between amorphous and crystalline states of GST plays the leading role in the 7



memory switching, while the small amount of Ag dopants promotes the crystallization of GST.24,25 However, heavily doped Ag-GST (> 10%) with a selective switching behavior hasn’t been reported yet. To reveal the fine details of the mechanism behind, it is of critical importance to figure out the roles of Ag and GST in the high-nonlinear, low-leakage selective switching, respectively. We conducted transmission electron microscopy (TEM) analysis based on a 100-time cycled vertical device to reveal the microstructure change of the heavily doped Ag-GST, as shown in Figure 4. To avoid any influence on the physical situation of Ag, low beam current was applied during TEM sample preparation. From the cross-section view of the switching layer, two distinctly different regions were observed: one region with uniform contrast and the other one with imbedded dark contrast clusters. It is interesting to find that the dark contrast clusters contact with either the TE or BE, but none bridges TE and BE together. High angle annular dark field (HAADF) and Electron Dispersive X-ray Spectroscopy (EDX) measurement were performed to further analyze the composition of the switching layer, as shown in Figure 4(c)(d). In HAADF mode, the image intensity is proportional to the atomic weight of the species.26 So the clusters with brighter contrast can be identified as Ag-rich residues after selective switching. The EDX mapping results further confirm that dense Ag signals are localized around the cluster region while sparse Ag signals are distributed evenly in the diffusing region. High resolution TEM (HRTEM) was also employed to examine the lattice structure for both cluster region and diffusing region (see Figure S5, supporting information). No lattice fringe was observed, indicating that the switching layer maintains in amorphous phase during the cycle testing. For comparison, we also conducted TEM analyses based on a pristine device (see Figure S6, supporting information). The switching layer of pristine device shows uniform contrast, and no Ag-rich cluster can be identified. Based on the TEM 8


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results, we can preliminarily infer that the rich Ag atoms in the GST glassy matrix should be a crucial factor that determines the selective switching characteristics. The TEM results provide the static evidence of Ag-rich filament residue after cycle testing. It is still crucial to describe the dynamic process for in-depth understanding of the selective switching, especially the low-to-high OFF switch dynamic, which marks the key difference between selective and memory switching. In order to describe the formation and diffusion dynamics of the rich Ag in GST glass, we fabricated a lateral device with Ag/Ag-GST/Ag structure, as shown in Figure 5(a). For observation convenience, a 6 µm gap between the lateral electrodes was adopted for testing (see Fgiure 5(b)). Because the lateral device has much larger distance between the two electrodes comparing to that of the vertical device (100 nm), it may require a larger quantity of Ag to form the observable conductive filament. However, if we increase the Ag concentration in the switching layer, the initial resistance will decrease, which will affect the switching behavior, as mentioned in Figure S1. Thus, Ag electrodes are employed to ensure sufficient supply of Ag atoms. I-V characteristics were measured based on the lateral device (see Figure 5(c)), showing reliable selective switching with similar Vth and ON/OFF ratio of the vertical structure. Based on the lateral device, we directly observed and recorded the selective switching process (See Supplementary Movie). Applying a small voltage bias (< 1 V), the current through the selector maintained at a low level (~ 10-10 A). However, by element mapping and EDX line scan profile characterization, it was found that Ag atoms were driven from the anode (right) to the cathode (left) with a gradient distribution, as shown on the left side of Figure 5(d). The highest atomic percent of Ag in the Ag-rich region exceeds 70%. When the applied voltage polarization was reversed, Ag atoms migrated from the left side to the right side with a mirror-like distribution, as presented on the right side of Figure 5(d), indicating the electric field induced Ag migration is 9



reversible and dependent on the polarization. When the electrical bias was continually increased, Ag-rich conductive filament starts to form near the cathode and grows towards the anode (See Supplementary Movie). When Vth was reached, the current abruptly increased by several orders of magnitude to 3 × 10-5 A and the selector was switched to ON state. Subsequently, we gradually reduced the voltage bias and observed a diffusion process of Ag filament. When Vhold was reached, the current sharply dropped to about 10-10 A and the selector switched back to OFF state. From the scanning electron microscope (SEM) image and EDX mapping results of the OFF state in Figure 5(e) and (f), a Ag-rich filament residue was observed. The dynamic process indicates that the selective switching of lateral device is highly associated to the electroforming and diffusion process of the Ag-rich conductive filament. We further conducted an experiment by applying a 3 V voltage to the lateral device for a longer time. It was found that the device maintained at a low resistance state, exhibiting as a memory switching. A robust Ag-rich filament bridging the two electrodes was observed (see Figure S7, supporting information), which accounts for the memory switching. This finding also provides a potential explanation for the earlier observed recovery of selective switching from a failure in Figure S3, in which thermal treatment can promote the Ag diffusion and break the bridging filament between the electrodes. The experiments based on lateral device demonstrated the fast formation and diffusion behavior of Ag filament in glassy GST material, which can be a strong supplement to the static TEM results. And more importantly, this experiment also distinguished selective switching from memory switching. For selective switching, the metallization filament should not be very strong, and can diffuse spontaneously under low electric field. While for memory switching, the filament should be strong enough to acquire non-volatile behavior and long data retention. According to the above experimental results, it is important to further examine the migration 10


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and diffusion dynamics of Ag in glassy GST. Glassy GST (Ge:Sb:Te = 2:2:5) with overstoichiometry of chalcogen is beneficial for Ag ionization and migration. More detailly, with a low average coordination number (less than 4)27, amorphous GST contains a large number of non-bonded anionic defects, through which Ag ions can easily move from one position to another (see Figure S8(a), supporting information). Applying Anderson-Stuart model, glassy GST, a p-type semiconductor, can be considered as an electrolyte, in which Ag atoms capture prevailing free carriers (holes) and form Ag+ ions (Ag + h+ = Ag+). The highly mobile Ag+ ions tend to react with non-bonded Te (NBT) atoms (Ag+ + Te2- = AgTe- or Ag+ + (Ge, Sb)Te- = Ag(Ge, Sb)Te) and may migrate among available equivalent sites by overcoming an activation energy (See Figure S8(b), Supplementary Information). The activation energy of glassy GST that hinders Ag+ migration under electric field arises from two parts related to NBT and bonded Te (BT), respectively. One is the coulombic energy barrier, resulted from the interactions between negatively charged NBTs and Ag+ ions; The other one is the inherent strain energy generated by BTs that the Ag+ must overcome before migrating through different sites. Having a longer bond length (2.77 Å)28 that corresponds to weaker bond strength, Ag-Te bond can easily break and recombine comparing with other Ag-chalcogenide bonds (See Table S1, Supplementary Information), indicating small coulombic trapping energy.4 It has also been demonstrated that the NBT defects in glassy GST are plentiful and closely spaced, suggesting a small strain energy barrier due to less amount of BTs.29 The low strain energy barrier and low coulombic energy barrier result in relatively low activation energy of glassy GST, which can facilitate the migration of Ag+ ions possessing a high ion mobility. With the application of an external voltage bias, Ag+ ions tend to migrate from anode to cathode and recombine with electrons (Ag+ + e- = Ag), forming Ag-rich region near the cathode. Besides the migration process under the electric field, Ag also diffuse and 11



dissolve spontaneously in GST opposite to the concentration gradient. In conjunction with the thermal diffusion driving force induced by the concentration gradient, the p-n junction between Ag-deficit region and Ag-rich region also contribute to the diffusion process, which forms a built-in field and pulls the Ag+ ions into the Ag-deficit region.30 The low energy barrier environment provided by glassy GST is another key requirement trigging selective switching behavior rather than non-volatile memory switching. Based on the Ag migration and diffusion dynamics associated to the measured I-V characteristics, we propose a switching model for the selector as shown in Figure 6. Initially, Ag atoms distribute isotropically and evenly in glassy GST. When a voltage potential is applied between TE and BE, Ag atoms will ionize and Ag+ ions begin to migrate along the electric field direction by overcoming the energy barrier in glassy GST matrix. Recombining with electrons near the cathode, a gradient distribution of Ag atoms is formed in parallel to the electric field direction (Figure 6(a) 1 and 4). Due to disorder-induced localized electronic states, amorphous GST exhibits a very low, trap-limited carrier mobility. Thus, ionic current plays the leading role at this stage, resulting in the ultra-low leakage current under low electric field. Continually increasing the applied voltage, the higher concentration at the cathode side gives the possibility to the electrodeposition of Ag-rich conductive filament. The upper solubility limit for Ag in glassy Ge-containing chalcogenides was determined to be roughly as large as 30-40 at.% at room temperature, referred to the total number of the cations.31 The up to 70 at.% Ag concentration near cathodic electrode caused the local supersaturation of Ag. The supersaturation of Ag atoms and nonuniformity in the electrode topography will tend to promote localized nucleation and electrodeposition. Initial Ag nucleus may randomly form at the interface and will cause the change of electric field distribution. In turn, concentrated electric field will accelerate the Ag migration and clustering towards initial 12


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nucleus, which will consume the local ion supply and effectively starve the other nuclei. A filament will grow from the cathodic electrode side and connect the TE and BE, resulting in the switch ON process (Figure 6(a) 2 and 5). At this stage, the conductive current of the Ag filament predominated, causing the several orders of magnitude increase of total current. When we reduce the applied bias to Vhold, the external electric field is not adequate to maintain the electrodeposition of filament. With the aid of chemical potential gradient and electrolyte non-stoichiometry, the Ag conductive filament rapidly diffuses into GST electrolyte in a very short time (Figure 6(a) 3 and 6). Eventually the selector goes back to high resistance state, showing as switch OFF process. It should be noted that the electrodeposition and diffusion are two competition processes, both of which play important roles in the selective switching. With larger voltage applied, the electrodeposition process competes ahead, and conductive Ag-rich filament start to grow from cathodic electrode to anodic electrode, corresponding to the switch ON at Vth. When we reduce the applied voltage, diffusion process predominant the dynamics, and the conductive Ag-rich filaments dissolve into GST again, corresponding to the switch OFF at Vhold. Until now, using Ag migration and diffusion dynamic, we addressed the whole process of the hysteresis selective switching.

4. Conclusion In this work, we have demonstrated exceptional selective switching characteristics based on heavily doped Ag-GST material. Compared to other reported access devices, our selector shows several advantages in terms of bidirectional operation, > 109 ON/OFF ratio, large hysteresis window, < 10 fA ultra-low leakage current, high nonlinearity, and simple structure, which make it a suitable candidate of universal selector for large-scale NVM and neuromorphic network implementation. For the first time, we studied the selective switching 13



behavior of heavily doped Ag-GST material, and gave the direct evidence by conducting a dynamic analysis based on a lateral device. The mechanism of selective switching can be attributed to the fast electrodeposition and diffusion dynamics of Ag in glassy GST material. Based on solid experimental evidences, a fine detailed switching model was put forward, which gives a better understanding for the similar EM based devices. With different switching mechanism from light doping condition, the volatile selective switching behavior provide new application directions and developable space for Ag-GST materials.

Associated content Supporting Information Supporting Information is available on the ACS Publications website. Threshold voltage and OFF state current depending on Ag concentration characteristics; I-V sweeping characteristics with different compliance current and 1 selector 1 resistive memory (1S1R) implementation; 6 × 6 selector array demonstration and corresponding device to device variability; Cycling induced breakdown and recovery by thermal treatment; HRTEM images of the Ag clustering region and Ag diffusing region; TEM and EDX mapping analysis based on a pristine device; SEM image for shorted lateral device; molecular dynamics model and Anderson-Stuart model for glassy GST material; Bond lengths of Ag-S, Ag-Se, Ag-Te in Ge-chalcogenide glass materials; supplementary movie provides the selective switching process in the lateral device.

Acknowledgements X. Ji and L. Song contributed equally to this work. We would like to thank Dr. Chao Wang for the valuable discussion during the revising process.

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Jo, S. H.; Kumar, T.; Narayanan, S.; Lu, W. D.; Nazarian, H. and Clara, S. 3Dstackable Crossbar Resistive Memory based on Field Assisted Superlinear Threshold (FAST) Selector. Electron Devices Meet. (IEDM), 2014 IEEE Int. 2014, 408, 3-5.

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Kim, E. C. S.; Kim, Y. B.; Kim, K. M.; Kim, S. J.; Lee, S. R.; Chang, M.; Cho, E.; Lee, M.; Lee, D.; Kim, C. J.; Chung, U. and Yoo, I. K. Performance of Threshold Switching in Chalcogenide Glass for 3D Stackable Selector. VLSI 2013, 6, T240.

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Shenoy, R. S.; Burr, G. W.; Virwani, K.; Jackson, B.; Padilla, A.; Narayanan, P.; Rettner, C. T.; Shelby, R. M.; Bethune, D. S.; Raman, K. V.; BrightSky, M.; Joseph, E.; Rice, P. M.; Topuria, T.; Kellock, A. J.; Kurdi, B. and Gopalakrishnan, K. MIEC (mixed-ionic-electronic-conduction)-based access devices for non-volatile crossbar memory arrays. Semicond. Sci. Technol. 2014, 29, 104005.

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Woo, J.; Lee, D.; Cha, E.; Lee, S.; Park, S. and Hwang, H. Control of Cu conductive filament in complementary atom switch for cross-point selector device application. IEEE Electron Device Lett. 2014, 35(1), 60-62.

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Luo, Q.; Xu, X.; Liu, H.; Lv, H.; Gong, T.; Long, S.; Liu, Q.; Sun, H.; Banerjee, W.; Li, L.; Lu, N. and Liu, M. Cu BEOL compatible selector with high selectivity (> 107), extremely low off-current (~ pA) and high endurance (> 1010). in Electron Devices Meet. (IEDM), 2015 IEEE Int., 2015, pp. 10–14.

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Figure 1. Device structure of the bidirectional selector (a) Top view, and (b) cross section. (c) I-V curve of the selector device, exhibiting bipolar characteristics with good nonlinearity and high ON/OFF ratio. (d) High precision measurement with slow sampling rate at low electric field. The ultra-low leakage currents (< 10 fA) near 0 V. Inset: fast sweep with high precision.

Figure 2. Switching character as a function of pulse width and amplitude for both directions:. The pulse amplitude varies from 2.0 V to 3.2 V for positive operation and -2.0 V ~ -3.2 V for negative operation. And applied pulse widths are 100 µs (a) – (b), 10 µs (c) – (d), 1 µs (e) – (f), respectively. The black frames mark the turn on voltages. 18


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Figure 3. (a) Pulse endurance cycling performance. The reversible switching up to 105 for positive and 3×104 for negative direction were demonstrated. (b) I-V sampling during endurance test. The switching characteristic remains stable during 104 cycling.

Figure 4. (a) Cross-sectional TEM image of the Pt/Ag-GST/TiW selector. (b) Enlarged bright field image of the two interested regions: Ag clustering region marked in red rectangle, and Ag diffusing region marked in blue rectangle. (c) Cross-sectional High Angle Annular Dark Field (HAADF) images of the two interested regions. (d) EDX mapping images of Ag of the two interested regions.

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Figure 5. (a) Schematic drawing to depict the lateral Ag/GST/Ag device structure and experimental set-up. (b) The top view SEM image of lateral device. The orange line denotes the EDX line scan direction. (c) I-V curve of the lateral structure. (d) EDX line scan profiles of the gap between the two lateral electrodes after 0.5 V electric field applied: left side from B to A, right side from A to B. SEM image (e) and EDX mapping (f) capturing the dynamic Ag migration in the lateral structure after selective switching.

Figure 6. (a) Schematics to show the formation and dissolution of Ag conductive bridge at various input voltages. (b) Typical I-V curve of Ag-GST selector. For the sake of clarity, the numbers correspond to the numbers in (a), The currents of the ON-switch (1-2 and 4-5) and the OFF-switch (2-3 and 5-6) are labelled, respectively. The arrows indicate the voltagesweep directions.

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